In many optical scanning systems an oscillating mirror is employed between a focusing lens and an image plane to scan the optical axis of the focusing lens across the image plane. Such apparatus may be employed for example to provide the relatively slow page scan in a raster scan apparatus where the relatively fast line scan in provided by a rotating polygon or hologon scanner.
A major problem associated with such an oscillating mirror scanner is that as the mirror is rotated, the point of focus of the optical system defines a circular arc that does not lie in a plane. For example, if the mirror pivots or rotates about a fixed axis lying in the plane of the mirror, the point of focus will describe a circular arc that will intersect the plane of focus, at most at only 2 points.
One solution is to rotate the mirror about an external pivot point (see U.S. Pat. No. 4,155,620 issued May 22, 1979 to Rawson). By locating the external pivot point correctly, at most three locations along the scan line can be kept in focus, for example, the mid-point of the scan and two points near the ends of the scan. However, focus error occurs between these points.
To maintain focus across an entire scan, the oscillating mirror must both rotate and have its pivot point translate during the scan (see U.S. Pat. No. 3,222,453 issued Dec. 7, 1965 to Whitesell). In the arrangement described by Whitesell, the pivot point of the mirror axis is translated in a direction parallel with the optical axis of the lens. This requires that the mirror be moved in nonlinear reciprocating motion. To provide this motion, a precision ground cam is required, that is difficult and expensive to produce.